Air Quality Index

The U. S. Environmental Protection Agency (EPA) has provided a scale called the Air Quality Index (AQI) for rating air quality. This scale is based on the National Ambient Air Quality Standards (NAAQS) and is described in the Code of Federal Regulations, Part 58, Appendix G. This report is based on the AQI standards. More information on the AQI can be found on the EPA's AirNow web site.

The highest AQI value and associated average (1-hour, 8-hour, or 24-hour) that is responsible for the AQI rating are highlighted bold in the table.

PM-2.5 is measured at local pressure and temperature conditions.

(1) - Not all monitors are reporting for this metropolitan area.

(2) - This is an average since midnight for the current day and does not represent an entire day of data.The reported overall AQI category could change and will not be complete until all the data for the calendar day is polled and processed and could be further subject to later change by the data validation process.

(3) - The 8-hour average shown for ozone is based on a surrogate value derived from the current hourly average.

† - No AQI has been calculated for this metropolitan area. This can happen early in the morning before enough valid ozone or carbon monoxide hourly samples have been collected to create an eight-hour average and should clear up once enough samples are collected.If there are no measurements for any parameter this may indicate a data collection problem. Once any data collection problem is resolved, the data will be filled in and an AQI will be calculated (if possible).

§ - No data of this type was collected on August 17, 2017 for this metropolitan area, or there were not enough valid hourly samples collected on this date to meet data completeness requirements of a minimum of 18 valid hourly samples in a day,or there have not been enough valid ozone or carbon monoxide hourly samples collected to create an eight-hour average.

§§ - There have been no hourly averages of this parameter collected for August 17, 2017 in this metropolitan area.

* - There is no AQI associated with hourly ozone averages less than 0.125 ppm (125 ppb).

PLEASE NOTE: Data in this table is collected from Virginia DEQ air monitoring sites, local agencies, and private monitoring networks. Site specific information is available for each metropolitan area by clicking on the metropolitan area name. This data has not been verified by the Virginia DEQ or the responsible entity and may change. While this is the most current data, it is not official until it has been certified by the appropriate technical staff. This table is updated hourly. Click here for information about all the monitoring sites.

The table above lists the current peak concentration for the critical pollutant, the corresponding Air Quality Index (AQI) category, and the AQI ratings for each of the NAAQS pollutants that are measured real-time. The critical pollutant is the pollutant with the highest AQI rating measured in the area. The table is updated each hour and covers the period from midnight through the indicated ending time for today's data or from midnight to midnight on other days. Listings are provided for each metropolitan area where pollutant levels are monitored by Virginia DEQ.

Interpreting the AQI

Each NAAQS pollutant has a separate AQI scale, with an AQI rating of 100 corresponding to the concentration of the Federal Standard for that pollutant. Additional information about the AQI and how it can be used is available from the EPA's AirNow web site.

Place your mouse pointer over the scale displayed above to view information about the Air Quality Index, and each of the rating levels.

The actual index calculation is different for each parameter measured and is specified by the EPA. The following table shows the various breakpoints used in calculating the AQI.

AQI Breakpoint Definitions

AQI Range

1hr Ozonein ppm

8hr Ozonein ppm

8hr Carbon Monoxidein ppm

1hr Sulfur Dioxidein ppm

24hr Sulfur Dioxidein ppm

1hr Nitrogen Dioxidein ppm

24hr PM-2.5in µg/m³ LC

0 - 50

Not Defined

0 - 0.054

0 - 4.4

0 - 0.035

Not Defined

0 - 0.053

0 - 12.0

51 - 100

Not Defined

0.055 - 0.070

4.5 - 9.4

0.036 - 0.075

Not Defined

0.054 - 0.1

12.1 - 35.4

101 - 150

0.125 - 0.164

0.071 - 0.085

9.5 - 12.4

0.076 - 0.185

Not Defined

0.101 - 0.36

35.5 - 55.4

151 - 200

0.165 - 0.204

0.086 - 0.105

12.5 - 15.4

0.186 - 0.304

Not Defined

0.361 - 0.649

55.5 - 150.4

201 - 300

0.205 - 0.404

0.106 - 0.200

15.5 - 30.4

Not Defined

0.305 - 0.604

0.65 - 1.249

150.5 - 250.4

301 - 400

0.405 - 0.504

Not Defined

30.5 - 40.4

Not Defined

0.605 - 0.804

1.25 - 1.649

250.5 - 350.4

401 - 500

0.505 - 0.604

Not Defined

40.5 - 50.4

Not Defined

0.805 - 1.004

1.65 - 2.049

350.5 - 500.4

500+

Not Defined

Not Defined

Not Defined

Not Defined

Not Defined

Not Defined

Not Defined

The AQI for ozone is based on either the peak eight-hour running average since midnight OR the peak one-hour measurement since midnight. When calculating air quality for the current day, the Virginia DEQ uses a surrogate 8-hour average based on the current hourly average.

The AQI for carbon monoxide is based on the peak eight-hour running average since midnight.

The AQI for sulfur dioxide is based on the peak one-hour measurement since midnight. There is a secondary NAAQS based on a 24-hour average of sulfur dioxide sampled from midnight to midnight, but this NAAQS has such a high starting point that is is never expected to be reached.

The AQI for nitrogen dioxide is based on the peak one-hour measurement since midnight.

The AQI for PM-2.5 is based on a 24-hour average sampled from midnight to midnight.

PLEASE NOTE: This data has not been verified by the Virginia DEQ and may change. This is the most current data, but it is not official until it has been certified by our technical staff. Data is collected from Virginia DEQ ambient monitoring sites and may include data collected by other outside agencies. This data is updated hourly. All times shown are in local standard time unless otherwise indicated.